In many spectroscopy applications, it is desirable to measure the spectra of a number of narrow spectral regions with high resolution; the narrow spectral regions being distributed more or less at random throughout a much broader overall spectral region. These narrow spectral regions are often referred to as spectral lines. For example, in absorption, emission or light scattering spectroscopy, the total region spanned by all sections of interest may be several thousands of Angstroms (100's of nanometers), while spectral measurements are actually needed for one or more narrow spectral regions having a span of less than one Angstrom at a resolution of the order of 0.1 Angstrom in the vicinity of particular spectral lines.
The total number of measurements wanted may be rather large. A certain amount of time is generally needed to make each measurement with adequate precision and there is the additional need to search out the appropriate spectral region for each line. As a result, the total amount of time required to scan, locate spectral regions, and carry out measurements can become quite large.
To deal with this type of application, there are commercially available two basic forms of dispersive spectrometers. The scanning spectrometer sweeps all of the wavelengths of the spectrum using a single detector to determine the presence of spectral lines in the sample being analyzed. The scanning spectrometer is slow and can only sense one narrow spectral band at one time. Another form of spectrometer has a plurality of detectors which can simultaneously select spectral energy lines in regions of interest. This type of spectrometer sometimes uses a separate exit slit photodetector and amplifier channel for each of the spectral energy lines to be measured. Some scanning is generally needed to examine the spectral region surrounding each line. For a given spectral resolution, the first method is slow and the second method expensive and not versatile.
An improvement in dispersive spectrometers is disclosed in U.S. Pat. No. 3,700,332. It is directed to an absorption spectrometer which involves the use of a controlled vibrating chopping mask. A photoelectric detector receives stepwise a series of chopped line and band flux intensities from different selected portions of the spectrum and generates a group of electrical values, each porportional to the total flux transmitted through each array. A Hadamard matrix is used to analyze the light flux for each spectral line. This system involves several moving components including the vibrating chopping mask. Thus, the system must be isolated from surrounding ambient vibration to provide the needed accuracy. It also requires the complex mathematical Hadamard analysis.
A correlation absorption spectrometer is disclosed in U.S. Pat. No. 3,955,891. The spectrometer has first and second spatial filters with a chopping device to alternately block light from the first filter onto two filter portions of the second filter. A weighted linear combination of the intensities of the selected wavelength interval of light from the light source is used. By way of a Hadamard matrix, the results are analyzed.
Another improvement in dispersive spectrometers is described in U.S. Pat. Nos. 3,752,585 and 4,049,353 and Canadian Pat. No. 896,652 which relate to various cross-dispersive spectrometer configurations using an Echelle grating for providing high resolution of the spectral energy lines. A rotatable prism is used in conjunction with a movable Echelle grating to provide a two dimensional spectral energy distribution in a rectangular focal plane. A cassette is used with an encoding disc to determine if spectral energy lines are present for atoms corresponding to the position of openings in the cassette. According to the improvement of U.S. Pat. No. 4,049,353, the cassette is used in conjunction with a two dimensional array of photomultiplier tubes to determine if spectral energy lines are present at the opening in the cassette. The use of a movable prism and Echelle grating in conjunction with the cassette requires extreme precision in set-up to avoid inaccuracy in readings. The machine is also very susceptible to vibration caused by the surroundings. The unit is not readily interchangeable to sense various elements because the cassette positioning has to be very accurate to decipher all the spectral energy information arriving at the cassette. This makes the cassette very expensive. The use of cross-dispersion also limits the allowable entrance slit height on this spectrometer. In turn, this reduces the light gathering capability of the instrument. A similar type of system is disclosed in U.S. Pat. No. 4,391,523 to Leeman Labs Inc.
There also exists cascade spectrometer arrangements where the first monochromator is used to select a single spectral region of interest and pass that single spectral region to a second spectrometer which disperses that single region to analyze specific spectral energy lines of the single spectral region. The advantage is that the first monochromator selects a narrow spectral region which is then resolved in higher resolution by the second spectrometer. The disadvantage is that only one narrow spectral region can be selected at any given time. Therefore, to determine spectral information in widely separated parts of a larger spectral region, the two monochromators must be scanned. This method is both slow and expensive.